The invention relates to a method and to an apparatus for the insertion of a weft thread into a shed of a rapier weaving machine as well as to a rapier weaving machine having such an apparatus or equipped to carry out such a method.
In rapier weaving machines the weft thread is inserted into a shed by means of a bringer rapier secured to a rapier rod or to a flexible band and is taken over at a transfer position in the central part of the shed by a taker rapier and transported further. The bringer rapier has the task of reliably taking up the weft thread that is presented, of inserting it into the shed and of leading it precisely to the taker rapier. Each of the rapiers has a rapier head with a thread clamp in order to fixedly clamp the weft thread during the weft insertion. With automatically clamping thread clamps the thread take-up takes place by pulling the weft thread into or out off preset clamping zones of the respective thread clamps.
When coarse and fine weft yarns are inserted one after the other it can happen that the coarse weft yarns, which require a greater clamping force, are fixedly clamped during the take-up at the inlet of the thread clamp where the clamping force is low so that the coarse weft yarns are only weakly held, whereas the fine weft yarns, which require a smaller clamping force, are drawn further into the thread clamp where they are correspondingly strongly held. In both cases operational disturbances can arise in that the coarse weft yarns are too weakly held during the weft insertion while the fine weft yarns are held too strongly during the transfer to the taker rapier and can tear.
For the manufacture of fabrics having weft yarns of different thicknesses or of different smoothness, controlled thread clamps can be used in one or in both rapier heads, with the thread clamp of the bringer rapier being actively opened during the thread take-up and that of the taker rapier being actively closed during the thread take-up.
An electromagnetically controlled thread clamp having an electromagnet as an actuator is for example described in the publication EP 0 690 160 A1. The thread clamp described in EP 0 690 160 A1 for the bringer rapier includes a movable clamping part which is held closed by means of a pre-stressed spring. For the opening of the clamping part a magnetizable part of the clamping part is drawn downwardly by the electromagnet which is arranged beneath the lower shed so that the take-up or transfer of the weft thread takes place with an open thread clamp. The disadvantages of this arrangement are that the construction of the arrangement and the control of the electromagnet are comparatively complicated and that the electromagnet requires a relatively high current which can lead to undesired heating up of the latter.
A further solution lies in the use of a cutting apparatus with a controlled thread clamp device, such as is for example disclosed in FIGS. 6 to 8 of EP 1020550 A1. The thread clamping device disclosed there includes a linear motor which permits the point in time of the release of the thread clamping device, and thus the point in time for the cutting of the weft thread, to be precisely controlled. For fine weft yarns the cutting time point can be advanced so that these are fixedly clamped at the inlet of the thread clamp whereas, for coarser weft yarns, the cutting time point can be delayed so that these are drawn further into the thread clamp.
Since the linear motor of the thread clamping device has to react at an insertion rate of 600 wefts/minute and above within less than 1 ms, comparatively strong and correspondingly expensive linear motors are required which moreover tend to overheat in operation.